In a TTL automatic flash (through-the-lens light measurement type flash light control) photographing system in which a flash light emission is stopped by a flash stop signal produced by a photographic camera when the value of the integrated amount of the light emitted from the flash device, reflected by the photographic object, and in turn having passed through the diaphragm aperture of the camera, reaches a predetermined value; a maximum camera:to-object distance range which can be covered by the maximum light quantity of the flash device for effecting a correct exposure of the photographic object (referred to as the flash-available distance range) is varied depending on the actual diaphragm aperture size for the camera exposure operation. (see, for example U.S. Pat. No. 4,359,275)
Conventional TTL automatic flash photography systems can not indicate such flash-available distance range since the conventional flash devices are not provided with means for receiving data of the actual diaphragm aperture size of the camera.
There are other known types of flash light photography systems in which the light from the photogaphic object is measured by light measuring means provided in the flash device and the flash light is stopped when the integrated value of the output of the light measuring means reaches a predetermined value, while the maximum light quantity data of the flash device is transferred to the camera. (see, for example Japanese patent application laid-open No. 54-158923) In this type of flash light photographing system, the maximum flash-available photographing distance is calculated by the maximum light quantity and the diaphragm aperture value calculated or manually set and transferred to display means for displaying the maximum photographing distance in the view finder of the camera.
This type of flash light photographing system has such drawbacks as follows:
(a) the size of the characters for the distances displayed in the view finder is too small to read;
(b) in order to read the flash-available distance range on the flash device, the operator must adjust a calculating plate to a position corresponding to the diaphragm aperture value set in the camera, then he must read the flash-available distance range; and
(c) since only the maximum photographing distance is displayed, the operator can not read the the minimum photographing distance of the flash-available distance range.
Other known flash devices are provided with display means for displaying the falsh available distance range in accordance with the calculation of the maximum photographing distance on the bases of a manually set aperture value and the maximum light value of the flash device. (see, for example, Japanese laid-open patent application No. 57-66429) In the flash device of this type, the diaphragm aperture value must be set both in the camera and the flash device. Therefore the setting of the diaphragm aperture value in the flash device may be overlooked.